Why are loopback addresses used to establish BGP peer relationships

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Loopback interfaces are logical interfaces. Compared with physical interfaces, loopback interfaces are not affected by links and can reduce the Border Gateway Protocol (BGP) flapping.

Other related questions:
What are the advantages of using the loopback address to establish BGP peer relationship
The loopback interface is a logical interface. Generally, it is in the Up state and is less affected by links than physical interfaces.

What are the advantages for BGP peer relationships established using loopback interface addresses on S series switches
For S series switches that support BGP, loopback interfaces are logical interfaces and are steadily Up in most cases. Compared with physical interfaces, loopback interfaces are not affected by links, improving BGP neighboring stability.

Problem and solution when BGP peer cannot be established
The BGP peer establishment on the firewall needs to use port 179 to establish TCP sessions and requires that OPEN messages be properly exchanged. Perform as follows to rectify the issue: 1. Check whether the AS number and IP address among peers are correct by using the display bgp peer command. 2. Check whether the router IDs configured on both BGP peers are conflicting by using the display bgp peer command. 3. If the loopback interface is used, check whether the peer connect-interface command is configured to specify the loopback interface as the source interface for sending BGP packets. 4. If EBGP neighbors are not directly connected to the physical layer, check whether the peer ebgp-max-hop command is configured. 5. Check whether there are available routes to the peer in the routing table. 6. Check whether there are reachable routes to the specified connect-interface by using the ping -a source-ip-address host-address command. 7. Check whether the ACL that is used to disable TCP port 179 is configured.

Meanings of BGP peer status
In addition to the common Idle and Established status, BGP peer also has the following status: 1. active: indicates that the TCP connection of the BGP session has not been established. 2. no neg: indicates that the negotiation is not performed. If IPv4 Unicast is configured at one end, and IPv4 Unicast and IPv4 Multicast are configured at the other end, after the peer is established, you can discover that IPv4 Unicast negotiation succeeds, and the BGP peer is in Established status. However, the IPv4 Multicast is in no neg status in that IPv4 Multicast is not configured at one end. 3. Idle (Admin): indicates the BGP peer is proactively disabled, and there is no attempt to establish it again. If the peer ignore command is executed, or this peer is set to be down through the MIB, this peer remains in this status.

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